1. Field of the Invention
This invention relates to steel wire for Metal Active Gas (MAG) welding in DC-electrode negative (hereinafter referred to as xe2x80x9cDC-ENxe2x80x9d), and to welding methods using the same. More particularly, the invention relates to steel wire for MAG welding in DC-EN which is used when the wire is connected to the negative terminal of a welding machine and steel sheets with a thickness of 0.2 to 4.5 mm are welded in one pass, and to a method for MAG welding in DC-EN using the steel wire.
2. Description of the Related Art
MAG welding uses a shielding gas containing a mixture of Ar gas and CO2 gas (5% by volume or more) or a mixture of Ar gas and O2 gas (1% to 10% by volume). It is a most commonly used welding method, is highly efficient, and it is widely used for the welding of steel materials. In particular, since automatic welding has come into widespread use rapidly, MAG welding has been widely used in the fields of shipbuilding, construction, bridges, automobiles, building machinery, etc. In the fields of shipbuilding, construction, and bridges, MAG welding is primarily used when thick steel sheets are subjected to high-current multi-pass welding. In the fields of automobiles and building machinery, MAG welding is often used when thin steel sheets are subjected to fillet welding.
In the fields of automobiles and building machinery, thin high-strength steel sheets have been increasingly used to reduce weight. However, as the strength of steel sheets is increased, strain and camber easily occur, resulting in an increase in the gap at the joint between the steel sheets. As the thickness of steel sheets as workpieces is decreased, the ratio of the gap between the steel sheets at the joint to the thickness of the workpieces increases. This results in an increase in the defect rate due to burn through. Under the circumstances, a new welding method in which burn through does not easily occur in thin steel sheets and superior gap weldability is exhibited has been desired.
In the conventional MAG welding process, welding in DC-electrode positive (hereinafter referred to as xe2x80x9cDC-EPxe2x80x9d), a wire is used as a consumable electrode and is connected to the positive terminal of a welding machine. This has been widely used because the arc is stable both in the low current region and in the high current region. In the welding process in DC-EP, a large amount of heat is generated due to the collision of positive ions having larger kinetic energy than electrons with the steel sheet on the negative side, and the steel sheet is deeply penetrated. Therefore, welding in DC-EP is suitable for multi-pass welding of thick steel sheets. However, when the welding process in DC-EP is employed for fillet welding of thin steel sheets, since the thermal effect on the steel sheets is large and the penetration into the steel sheets is deep, weld defects easily occur due to burn through. When thin steel sheets are subjected to fillet welding, it is most important to avoid weld defects due to burn through and to improve welding speed. Therefore, use of welding in DC-EP in fillet welding of thin steel sheets is problematic.
On the other hand, Metal Inert Gas (MIG) welding, which is a welding process in DC-EP using inert gas, such as pure Ar, is employed when some special steels are welded. For example, it is used for welding of high tensile strength steels having a tensile strength of approximately 980 Mpa, which require high toughness by decreasing the amount of oxygen in the weld metal and the welding of stainless steels. A method has been disclosed in which REM is added to the welding wire to stabilize the arc from a welding wire in MIG welding (Journal of Japan Welding Society Vol. 50 (1981) No. 11 Page 1066-1074). The advantage of adding stabilizing REM to the welding wire is that it is possible to suppress the shift of the cathode spot due to cleaning action in MIG welding, i.e., the phenomenon in which the cathode spot (on the steel sheet side) crawls around the periphery of the weld metal in search of a more electron-releasing oxide, and that stable spray transfer is enabled. However, adding REM to the wire in the MAG welding process, which contains oxidizing gas and which is active, coarsens molten particles and increases the number of large spatters.
On the other hand, in welding in DC-EN in which a wire is connected to the negative terminal of a welding machine, a small amount of heat is generated due to the collision of electrons having less kinetic energy than positive ions with the steel sheet on the positive side, and penetration of the steel sheets is shallow, while the welding speed of the wire is high and the amount of weld deposition is large. Therefore, welding in DC-EN is suitable for welding of thin steel sheets and, in particular, suitable for welding when a large gap occurs between steel sheets at the joint. However, in the welding process in DC-EN, molten particles suspended from the tip of the wire are coarsened, resulting in the arc instability. Furthermore, in high-speed welding, humping of weld beads and irregular bead shape tend to occur. Consequently, practical use of welding in DC-EN has not been implemented.
With respect to welding in DC-EN, several proposals have been made in limited fields. For example, Japanese Unexamined Patent Publication Nos. 58-167078 and 5-138355 disclose consumable electrode-type gas shielded arc welding methods in which welding is performed by controlling the ratio between the period of welding in DC-EN and the period of welding in DC-EP because there are great differences in the depth of penetration and the welding speed between welding in DC-EN and welding in DC-EP. However, in such welding methods, the arc stability is insufficient, and compositions of wires are not taken into consideration.
As described above, the welding process in DC-EN is suitable for welding thin steel sheets because of the shallow penetration and the large amount of weld deposition and, in particular, is considered to be suitable for welding of a joint having a large gap. However, in the conventional steel wire for welding, since coarse molten particles unstably suspend from the tip of the wire, the arc becomes unstable and a large number of spatters are generated.
It would, therefore, be highly advantageous to provide steel wire for MAG welding in DC-EN, which is suitable for use in welding in DC-EN, which prevents burn-through defects in the welding of thin steel sheets, which has superior gap weldability and arc stability so that sound welding of a joint with a large gap can be performed and the number of spatters generated is decreased. It would also be advantageous to provide a method for MAG welding in DC-EN using the steel wire.
We carried out thorough research on the influence of the composition of wire on arc stability, gap weldability, and the bead shape in welding in DC-EN. We discovered that the arc can be stabilized and the gap weldability is ensured by combining the welding process in DC-EN, which usually makes the arc unstable in MAG welding, with the addition of REM to the wire.
We found the following:
1) By adding rare-earth elements (atomic Nos. 57 to 71, hereinafter, referred to as xe2x80x9cREMxe2x80x9d), and by primarily adding Ce, to the wire, arc stop in the low voltage region can be prevented, and regular short circuit transfer can be performed.
2) By setting the contents of Si, Mn, Ti, Zr, and Al, which are deoxidizing elements, to satisfy predetermined ranges and relationships, stable gap weldability can be obtained.
Accordingly, in one aspect, the invention includes a steel wire for MAG welding in DC-EN containing about 0.20% by mass or less of C, about 0.25% to about 2.5% by mass of Si, about 0.45% to about 3.5% by mass of Mn, about 0.005% to about 0.040% by mass of REM, about 0.05% by mass or less of P, about 0.05% by mass or less of S, and the balance being Fe and incidental impurities.
In the steel wire of the invention, preferably, the D1 value in accordance with the following equation (1) is in the range of about 1.2 to about 2.1.
D1=((Si)/2)+(Mn)/3)xe2x80x83xe2x80x83(1) 
(Si): Si content (% by mass)
(Mn): Mn content (% by mass)
Preferably, the steel wire of the invention further contains at least one of about 0.30% by mass or less of Ti, about 0.30% by mass or less of Zr, and about 0.50% by mass or less of Al.
Preferably, the steel wire of the invention further contains about 3.00% by mass or less of Cr.
More preferably, the D2 value in accordance with the following equation (2) is in the range of about 1.2 to about 2.1.
D2=((Si)/2)+((Mn)/3)+((Ti)+(Zr)+(Al))xe2x80x83xe2x80x83(2) 
(Si): Si content (% by mass)
(Mn): Mn content (% by mass)
(Ti): Ti content (% by mass)
(Zr): Zr content (% by mass)
(Al): Al content (% by mass)
Preferably, the steel wire of the invention further contains about 0.0001% to about 0.0150% by mass of K.
Preferably, the steel wire of the invention includes a Cu-plating layer having an average thickness of about 0.5 xcexcm or more as a surface layer.
In another aspect of the invention, a method for MAG welding in DC-EN for welding steel sheets having a thickness of about 0.2 to about 4.5 mm using any one of the steel wires for MAG welding in DC-EN described above, includes performing single pass welding, wherein the ratio of the gap between the steel sheets to the thickness of the steel sheets is about 1/2 or more.